Electrographic imaging and developing processes have been extensively described in both the patent and other literature. Such electrographic imaging and development processes include the imaging and development processes of electrophotography, electrophoretic migration imaging and modulated electrostatic printing.
A typical electrophotographic process employs a photoconductive element comprising a coating of a photoconductive insulating material on a conductive support. The element is given a uniform charge in the dark and then is exposed to an image pattern of activating electromagnetic radiation such as white light or X-rays. The charge on the photoconductive element is dissipated in the illuminated areas to form an electrostatic charge pattern which is then developed by contact with a developer composition comprising a carrier and an electrographic marking material. The marking particles deposit on the surface bearing the electrostatic charge pattern, in accordance with either the charge pattern or the discharge patterns as desired.
In a typical electrophoretic migration imaging process, an imaging composition comprising electrostatic charge-bearing photoconductive particles, i.e. electrically photosensitive particles, is positioned between two spaced electrodes, one of which may be transparent. To achieve image formation in this process, the electrically photosensitive particles positioned between the two spaced electrodes are subjected to the influence of an electric field and exposed to a pattern of activating radiation. As a result, the electrically photosensitive particles are caused to migrate electrophoretically to the surface of one or the other of the spaced electrodes upon which an image of the pattern of activating radiation is defined by the electrically photosensitive particles. Typically, a negative image of the pattern is formed on one electrode, and a positive image of the pattern is formed on the opposite electrode.
One method of modulated electrostatic printing referred to in the patent literature comprises generating an ion stream in the direction of a print receiving medium, modulating the cross-sectional density flow of ions in the stream in accordance with a pattern to be reproduced, and introducing a cloud of substantially uncharged marking particles adjacent to print receiving medium whereby the modulated ion stream selectively collides with and induces charges on the marking particles in the cloud. The marking particles are then deposited on the printing receiving medium in accordance with the pattern being reproduced. The direction of the ion stream flow is determined by an electrical field. The ion stream is modulated in accordance with the charge pattern by a grid having computer addressed electronic gates.
Sometimes developed electrographic images have a low maximum density. This low maximum density can result from a variety of reasons. In the case of photoelectrophoretic imaging, it might relate to the nature of the pigment used in or as the electrically photosensitive component of the imaging composition, or the relatively large mass of electrically photosensitive marking materials which must be caused to migrate to establish optimum density. In the case of developing electrostatic images with liquid aerosol developers, the low maximum density might result from low solubility of the dye in the carrier. In other cases, the charge density defining the latent electrostatic image may be so low as to be unable to attract sufficient marking particles to obtain an optimum maximum density. The low charge density of the latent image can result from a variety of factors including insufficient exposure and/or insufficient insulating capability of the surface upon which the latent charge image is placed.
It is frequently desirable to increase the density of an already developed charge image.